Braking apparatus are employed to inhibit motion in various devices that have rotating members. One such device is a robot, which may employ a braking apparatus to inhibit motion of the robot arm.
In particular, robots typically include a robot arm to move a work tool between various locations where work operations are performed on a work piece. To move the robot arm, the robot includes a motor that provides rotational motion through a shaft. Linkages or other structures within the robot convert such rotational motion into desired movement of the robot arm. To stop the robot arm in a particular position, precise motor control is used. While precise motor control is typically sufficient to stop and hold the robot arm in position, a brake is also required by the robot to hold a robot arm in position either in an emergency stop or during a power interruption.
More specifically, upon the loss of electrical power due to the initiation of an emergency stop or an accidental power loss the robot arm must be stopped and held in position by means other than motor control. Stopping and holding the robot arm in position during the accidental loss of electrical power prevents the robot from damaging the work piece or injuring a technician working in close proximity to the robot arm. Because the control signals which cause the motor to hold the robot arm in position are absent without electrical power, it is necessary that a braking mechanism be employed to stop and hold the robot arm in the event of a power loss. The braking mechanism can also be used to stop and hold the robot arm in position when the electrical power is purposefully removed from the motor, such as during routine maintenance of the robot.
In a conventional braking apparatus, such as the disk brakes used in automobiles, a rigid brake pad is urged into contact with a rotating member attached to the shaft. Contact between the rigid brake pad and the rotating member creates a frictional force which slows the rotation of the rotating member and, thus, the shaft. In order to rapidly stop the shaft, a relatively large force must be applied to the brake pad. Additionally, to enable fail safe operation, this large force must be provided by a biasing member, such as a spring, which does not require electrical power. This biasing member is operable to urge the brake pad into contact with the rotating member in the event of a power loss.
The robot arm also includes a brake actuator which is operable to overcome the relatively large force of the biasing member when it is desired to allow movement of the robot arm. Because the brake actuator must overcome the large bias force, a relatively large and powerful brake actuator must be provided. Accordingly, one drawback to using a conventional braking apparatus is that a relatively large and bulky actuator must be carried in the robot arm, which undesirably increases the size and weight of the robot arm. Increasing the size of the robot arm inhibits the maneuverability of the robot arm in confined spaces, and may limit the type of work operations performed by the robot. Moreover, the increased weight of the robot arm may require a more powerful motor, thereby raising the cost of the robot.
What is needed therefore is an apparatus and method for braking a robot arm which rapidly stops and holds the robot arm in position during the loss of electrical power without significantly increasing the size or weight of the robot arm.